This invention relates to a fiber optic cable, and more particularly to a fiber optic cable wherein an optical fiber extends axially within a helically wound textile serve. The invention also relates to a method of manufacturing such a cable.
Fiber optic cables often comprise a concentric series of elements. A fiber optic element usually extends axially along the cable and serves as an optical wave path. The fiber extends within one or more concentrically arranged layers extending longitudinally along the cable. These layers are designed to provide protection and support for the relatively delicate optical fiber, as well as other properties desirable in such a cable such as flexibility, tensile, flexure and impact strength. A cable jacket is usually selected to render the cable flame and moisture resistant. Usually, a tube, or a strength member, or both are placed over the buffered or unbuffered fiber, and the jacket is extruded thereover to complete the cable and define its outside diameter.
One of the more economical methods of providing strength and protection to a relatively delicate optical fiber is to overlay the optical fiber with textile strength members in a serving operation. One major problem with cables of such construction resides in the maintenance of concentricity between the fiber and the serve, particularly during manufacture. When off center, the fiber has a tendency to conform to the helical path of the textile serve, aggravating a tendency toward microbending. Microbends result in a significant loss of light transmission. Furthermore, when concentricity is not maintained, it is more difficult to connect cables so that the optical fibers are aligned precisely. Assuring concentricity would make handling during manufacture easier, increase the durability of the product, and provide for more precisely aligned connections.
One remedy is to replace the serve with an extruded tube, but this approach is costly. Prior tubeless cables have approached the problem of microbending by serving the textile strength members directly over a heavily buffered optical fiber. The heavy buffering reduces a tendency for the helical path of the serve to induce microbending in the optical fiber. However, the heavy buffering increases production costs without improving the concentricity of the fiber within the serve. As indicated above, this lack of concentricity is a cause of microbending and, therefore, increased signal attenuation.
One possible remedy to loss of concentricity during manufacture would be to keep the fiber taut during the serving operation. If sufficient tension is placed on the fiber as the serve is being applied, and if this tension is maintained during all subsequent operations, the fiber can be kept on its center position. However, the required tension would be likely to damage or break many of the optical fibers now in use. Furthermore, residual tension in the fiber after cable manufacture can itself result in higher attenuation and poorer mechanical performance.
A major object of this invention is to provide an economical tubeless fiber optic cable with an improved means for assuring the concentricity of the optical fiber and a textile serve.
Another object of this invention is to provide an improved tubeless fiber optic cable which minimizes transmission losses due to microbending.
A still further object of this invention is to provide a method of manufacturing a tubeless cable with improved concentricity between the fiber and a serve.
Other objects and features of the invention will be apparent from the following description and from the drawings.